This invention relates to the field of selectable frequency generators. In particular, the invention relates to a circuit for producing pulses which simulate an engine. The output pulses may be used for calibrating control devices for such engine. For such applications the circuit output must be settable from frequencies on the order of 20 Hz to frequencies in excess of 10 KHz. When it is employed as a calibration unit, the frequency produced is desirably accurate to within 0.1%. To obtain this degree of accuracy requires that the circuit be capable of calibration in steps of 0.025%.
Given the above requirements, namely a digital frequency generator which is capable of producing frequencies up to 10 KHz and is settable to within 0.025% accuracy, a base frequency on the order of 40 MHz would be required if straight frequency scaling were employed. A base frequency of that magnitude, while attainable, is not practicable and would require an inordinate amount of expensive logic to implement.
A technique which produces the required degree of accuracy but avoids the problem of a very high base frequency is to provide the required degree of accuracy over a period of time. That is, short term accuracy (on the order of a few milliseconds) which could be obtained using a 40 MHz base frequency is traded off for long-term accuracy. The average output is made accurate to the required degree by increasing the periods of a certain fraction of the output pulses by one clock cycle. Where this "extended division" technique is used, a base frequency on the order of 2 MHz is sufficient.
For frequencies below approximately 500 Hz, the above "extended division" technique is not required. Accordingly, the present invention which is capable of the desired frequency range and accuracy has two modes of operation, one for producing output pulses in a first low frequency range by straight frequency division techniques and in a higher frequency range utilizing the extended division techniques.
It is accordingly an object of the present invention to provide a frequency generator which is capable of producing a pulse frequency output which is highly accurate.
It is another object of the present invention to provide a circuit capable of operating in either of two modes depending upon the desired frequency output.
It is a further object of the present invention to provide a computer controlled digital frequency generator capable of producing an output dependent upon the information received from the controlling computer.
It is a still further object of the present invention to provide a frequency generating circuit capable of producing a highly accurate output by comparing its operation rate against the selected rate and correcting for differences exceeding a threshold magnitude.
Other objects and advantages of the invention will become apparent from the remaining portion of the specification.